Identifying unbound strong bunching and the breakdown of the Rotating Wave Approximation in the quantum Rabi model

We use a recently derived gauge-invariant formulation of the problem of an incoherently-driven two-level system coupled to an optical cavity, to explore the transition between different coupling regimes -- in particular, between the weak, and the ultra-strong coupling regimes. We explore this transition using the second-order intensity correlation $g^{(2)}(\tau)$ of the emitted light, and find strong, unbounded bunching of the emission from systems governed by the Rabi Hamiltonian. Surprisingly, this effect is observed not only in the ultra-strong coupling regime, but also in the regime of coupling typically recognized as weak coupling, where the Jaynes-Cummings Hamiltonian predicts the opposite, strongly antibunched emission. This suggests that the intensity correlations are a particularly sensitive probe of the divergence between the Jaynes-Cummings and Rabi Hamiltonians, and can serve as an indicator of the breakdown of the rotating wave approximation

Theoretical description of light emission in the presence of nanoscale resonators: from classical scattering to photon states entanglement and statistics

Peer reviewe

Fano asymmetry in zero–detuned exciton–plasmon systems

Plasmonic resonances in metallic nanostructures can strongly enhance the emission from quantum emitters, as commonly used in surface-enhanced spectroscopy techniques. The extinction and scattering spectrum of these quantum emitter-metallic nanoantenna hybrid systems are often characterized by a sharp Fano resonance, which is usually expected to be symmetric when a plasmonic mode is resonant with an exciton of the quantum emitter. Here, motivated by recent experimental work showing an asymmetric Fano lineshape under resonant conditions, we study the Fano resonance found in a system composed of a single quantum emitter interacting resonantly with a single spherical silver nanoantenna or with a dimer nanoantenna composed of two gold spherical nanoparticles. To analyze in detail the origin of the resulting Fano asymmetry we develop numerical simulations, an analytical expression that relates the asymmetry of the Fano lineshape to the field enhancement and to the enhanced losses of the quantum emitter (Purcell effect), and a set of simple models. In this manner we identify the contributions to the asymmetry of different physical phenomena, such as retardation and the direct excitation and emission from the quantum emitter.Department of Education, Research and Universities of the Basque Government (IT-1526-22); Ministerio de Ciencia e Innovación and Agencia Estatal de Investigación (MCIN/AEI/10.13039/501100011033, PID2019-107432GB-I00); Predoctoral MCIN grant BES-2017-080073.Peer reviewe

Gold‐ and Silver‐Coated Barium Titanate Nanocomposites as Probes for Two‐Photon Multimodal Microspectroscopy

Improved multiphoton‐excited imaging and microspectroscopy require nanoprobes that can give different nonlinear optical signals. Here, composite nanostructures with a barium titanate core and a plasmonic moiety at their surface are synthesized and characterized. It is found that the core provides a high second‐order nonlinear susceptibility for sensitive second harmonic generation (SHG) imaging in living cells. As a second function in the two‐photon regime, the plasmonic part yields high local fields for resonant and nonresonant surface enhanced hyper Raman scattering (SEHRS). SEHRS complements the one‐photon surface enhanced Raman scattering (SERS) spectra that are also enhanced by the plasmonic shells. Barium titanate silver core–shell (Ag@BaTiO3) composites are specifically suited for SEHRS and SHG excited at 1064 nm, while gold at barium titanate (Au@BaTiO3) nanoparticles can be useful in a combination of SHG and SERS at lower wavelengths, here at 785 nm and 850 nm. The theoretical models show that the optical properties of the BaTiO3 dielectric core depend on probing frequency, shape, size, and plasmonic properties of the surrounding gold nanoparticles so that they can be optimized for a particular type of experiment. These versatile, tunable probes give new opportunities for combined multiphoton probing of morphological structure and chemical properties of biosystems.Peer Reviewe

Identifying unbound strong bunching and the breakdown of the rotating wave approximation in the quantum Rabi model

We use a recently derived gauge-invariant formulation of the problem of a two-level system coupled to an optical cavity to explore the transition between the weak and the ultrastrong coupling regimes of light-matter interaction. We explore this transition using the intensity correlations g(2)(τ) of the emitted light, and we find strong, unbounded bunching of the emission from systems governed by the Rabi Hamiltonian. Surprisingly, this effect is observed not only in the ultrastrong coupling regime, but also for weakly coupled systems, where the Jaynes-Cummings Hamiltonian would predict the opposite, antibunched emission. This suggests that the higher-order correlations are a particularly sensitive probe of the divergence between the Jaynes-Cummings and Rabi Hamiltonians, and they can serve as an indicator of the breakdown of the rotating wave approximation. Our findings indicate also that the boundary between the weakly, strongly, and ultra-strongly-coupled dynamics is much richer than currently accepted.Á.N., R.E., U.M., and J.A. acknowledge the financial support from the Spanish Ministry of Science and Innovation and the Spanish government agency of research MCIN/AEI/10.13039/501100011033 through Project Ref. No. PID2019-107432GB-100, and from the Department of Education, Research and Universities of the Basque Government through Project Ref. No. IT 1526-22. Á.N. acknowledges funding from MCIN/AEI/10.13039/501100011033 and “ESF Investing in your future” through Project Ref. No. BES-2017-080073. M.K.S. acknowledges funding from the Macquarie University Research Fellowship Scheme (MQRF0001036), and the Australian Research Council Discovery Early Career Researcher Award DE220101272.Peer reviewe

Scattering of classical and quantum states of light with angular momentum by dielectric micro/nanoresonators

Resumen del trabajo presentado a la NanoSpain Conference, celebrada en Madrid del 17 al 20 de mayo de 2022.Peer reviewe

Research data supporting "Characterizing the Backscattered Spectrum of Mie Spheres"

Each folder contains .txt files of the data for each of the figures indicated on its name, together with README instructions on each case.The file contains the dataset corresponding to the figures of the article "Characterizing the Backscattered Spectrum of Mie Spheres" written by Martín Molezuelas-Ferreras, Álvaro Nodar, María Barra-Burillo, Jorge Olmos-Trigo, Jon Lasa-Alonso, Iker Gómez-Viloria, Elena Posada, J. J. Miguel Varga, Rubén Esteban, Javier Aizpurua, Luis E. Hueso, Cefe Lopez, and Gabriel Molina-Terriza (DOI: 10.1002/lpor.202300665). The data is organized into different folders, and each folder contains .txt files of the data for each of the figures indicated on its name, together with README instructions on each case.PRE2018-085136. MCIN/AEI/10.13039 /501100011033 through Project Ref. No. FIS2017-87363-P. MCIN/AEI/10.13039/501100011033 and “ESF Investing in your future” through Project Ref. No. BES-2017-080073. MCIN/AEI/10.13039/501100011033 and “ERDF A way of making Europe” through Project Ref. No. PID2022-139579NB-I00. Department of Education, Research and Universities of the Basque Government through Project Ref. No. IT 1526-22. CSIC Research Platform PTI-001. MCIN/AEI/10.13039/501100011033 through Project Ref. No. MDM-2016-0618. MCIN/AEI/10.13039/501100011033 and the European UnionNextGenerationEU/PRTR through the Juan de la Cierva Fellowship Ref. No. FJC2021-047090-I. MCIN/AEI/10.13039/501100011033 and “ERDF A way of making Europe” through Project Ref. No. PID-2022-137569NBC43. MCIN/AEI/10.13039/501100011033 through Project Ref. No. PID2021-124814NB-C21.Peer reviewe